Electron Transfer Reactions

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B.8- B. 9 B.8- B. 9 In which you will learn about:In which you will learn about:

redox definitionsredox definitionsassigning oxidation numbersassigning oxidation numbers

half reactionshalf reactionsbalancing redox equationsbalancing redox equations

Electron Transfer ReactionsElectron Transfer ReactionsElectron Transfer ReactionsElectron Transfer Reactions

• Electron transfer reactions are Electron transfer reactions are oxidation-reductionoxidation-reduction or or

redoxredox reactions. reactions.

• Results in the generation of an electric current Results in the generation of an electric current

(electricity) or be caused by imposing an electric current. (electricity) or be caused by imposing an electric current.

• Therefore, this field of chemistry is often called Therefore, this field of chemistry is often called

ELECTROCHEMISTRY.ELECTROCHEMISTRY.

Terminology for Redox Terminology for Redox ReactionsReactions

• OXIDATIONOXIDATION—loss of electron(s) by a species; —loss of electron(s) by a species; increase in oxidation number; increase in increase in oxidation number; increase in oxygen.oxygen.

• REDUCTIONREDUCTION—gain of electron(s); decrease in —gain of electron(s); decrease in oxidation number; decrease in oxygen; oxidation number; decrease in oxygen; increase in hydrogen.increase in hydrogen.

• OXIDIZING AGENTOXIDIZING AGENT—electron acceptor; —electron acceptor; species is reduced.species is reduced.

• REDUCING AGENTREDUCING AGENT—electron donor; species —electron donor; species is oxidized.is oxidized.

• OXIDATIONOXIDATION—loss of electron(s) by a species; —loss of electron(s) by a species; increase in oxidation number; increase in increase in oxidation number; increase in oxygen.oxygen.

• REDUCTIONREDUCTION—gain of electron(s); decrease in —gain of electron(s); decrease in oxidation number; decrease in oxygen; oxidation number; decrease in oxygen; increase in hydrogen.increase in hydrogen.

• OXIDIZING AGENTOXIDIZING AGENT—electron acceptor; —electron acceptor; species is reduced.species is reduced.

• REDUCING AGENTREDUCING AGENT—electron donor; species —electron donor; species is oxidized.is oxidized.

44You can’t have one… without the other!

• Reduction (gaining electrons) can’t happen without an oxidation to provide the electrons.

• You can’t have 2 oxidations or 2 reductions in the same equation. Reduction has to occur at the cost of oxidation

LEOLEO the lion says the lion says GERGER!!

GER!GER!

Another way to remember

•OIL RIG

OXIDATION-REDUCTION REACTIONS

Direct Redox ReactionDirect Redox Reaction

Oxidizing and Oxidizing and reducing agents in reducing agents in

direct contact.direct contact.

Cu(s) + 2 AgCu(s) + 2 Ag++(aq) ---> (aq) ---> CuCu2+2+(aq) + 2 Ag(s)(aq) + 2 Ag(s)

OXIDATION-REDUCTION REACTIONS

Indirect Redox ReactionIndirect Redox Reaction

A battery functions by transferring electrons A battery functions by transferring electrons through an external wire from the reducing through an external wire from the reducing

agent to the oxidizing agent.agent to the oxidizing agent.

Practice Exercises Section 1Practice Exercises Section 1

• 1) Why is it that when something gains electrons, it is said to be reduced? What is being reduced?

• 2) Make a list of important vocabulary words from slides 1-7 and define each.

Assigning Oxidation #sAssigning Oxidation #s

1) Free elements are 0

2) Monatomic ions are their charge

3) Hydrogen is +1

4) Oxygen is -2

5) Group 1 elements are +1, Group 2 are +2, and Group 17 are -1

6) The sum of all oxidation numbers must equal 0

7) The sum of all oxidation numbers in a polyatomic ion must equal the charge of the ion

Oxidation #s ExamplesOxidation #s Examples

• Assign oxidation numbers for each element in Assign oxidation numbers for each element in the following compound:the following compound:

• Al2O3

– Al = +3 according to rule 2

– O = -2 according to rule 4

– 2(+3) + 3(-2) = 0 according to rule 6

• XeF4

– F = -1 according to rule 5

– x + 4(-1) = 0 according to rule 6

– Solving for x, tells us that Xe = +4

Assign oxidation states for all atoms in each of the following compounds:

a.KMnO4

b.NiO2

c.K4Fe(CN)6 [Fe only]

d.(NH4)2HPO4

e.P4O6

Balancing Equations Balancing Equations for Redox Reactionsfor Redox Reactions

Some redox reactions have equations that must be balanced by Some redox reactions have equations that must be balanced by special techniques.special techniques.

MnOMnO44-- + 5 Fe + 5 Fe2+2+ + 8 H + 8 H++

---> Mn---> Mn2+ 2+ + 5 Fe+ 5 Fe3+3+ + 4 H + 4 H22OO

Mn = +7Mn = +7 Fe = +2Fe = +2Fe = +3Fe = +3Mn = +2Mn = +2

Balancing Balancing EquationsEquations

Consider the Consider the reduction of Agreduction of Ag++ ions with copper ions with copper metal.metal.

Cu + AgCu + Ag++ --give--> Cu --give--> Cu2+2+ + Ag + Ag

1414Balancing Balancing EquationsEquations

Step 1:Step 1: Divide the reaction into half-reactions, one for Divide the reaction into half-reactions, one for oxidation and the other for reduction.oxidation and the other for reduction.

OxOx Cu ---> CuCu ---> Cu2+2+

RedRed Ag Ag++ ---> Ag ---> Ag

Step 2:Step 2: Balance each element for mass. Already done Balance each element for mass. Already done in this case.in this case.

Step 3:Step 3: Balance each half-reaction for charge by Balance each half-reaction for charge by adding electrons.adding electrons.

OxOx Cu ---> Cu Cu ---> Cu2+2+ + + 2e-2e-

RedRed Ag Ag++ + + e- e- ---> Ag---> Ag

1515Balancing Balancing EquationsEquations

Step 4:Step 4: Multiply each half-reaction by a factor so that Multiply each half-reaction by a factor so that the reducing agent supplies as many electrons as the the reducing agent supplies as many electrons as the oxidizing agent requires.oxidizing agent requires.

Reducing agentReducing agent Cu ---> Cu Cu ---> Cu2+2+ + 2e- + 2e-Oxidizing agentOxidizing agent 22 Ag Ag++ + + 22 e- ---> e- ---> 22 Ag AgStep 5:Step 5: Add half-reactions to give the overall equation.Add half-reactions to give the overall equation.Cu + 2 AgCu + 2 Ag++ ---> Cu ---> Cu2+2+ + 2Ag + 2Ag

The equation is now balanced for both The equation is now balanced for both charge and mass.charge and mass.

Specify which of the following are redox reactions. Identify the oxidizing agent, reducing agent, substance being oxidized, and the substance being reduced. Write half reactions to find out how many electrons are being transferred.

1)CH4 (g) + 2 O2 (g) CO2 (g) + H2O (g)

2)Zn (s) + 2 HCl (aq) ZnCl2 (aq) + H2 (g)

3)Cr2O72- (aq) + 2 OH- (aq) 2 CrO4

2- (aq) + H2O (l)

Tips on Balancing Tips on Balancing EquationsEquations

• Never add ONever add O22, O atoms, or , O atoms, or OO2-2- to balance oxygen. to balance oxygen.

• Never add HNever add H22 or H atoms to or H atoms to balance hydrogen.balance hydrogen.

• Be sure to write the correct Be sure to write the correct charges on all the ions.charges on all the ions.

• Check your work at the end Check your work at the end to make sure mass and to make sure mass and charge are balanced.charge are balanced.

• PRACTICE!PRACTICE!

Electron Transfer Reactions

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